Step 13.6: Select a Motor and Gearbox

The Kinematic Torque and Speed dialog-box opens.

Note: The first time you use this dialog in a session, MechDesigner loads the database of Motors and Gearboxes. Please wait for the database to open.

Select a Motor Manufacturer and a Motor Part Number:

The Servomotor selection-box is at the top and left of the dialog.

The box to its right should then list the gearboxes available from the Manufacturer

This is to the right of the Motor selection-box

To help you make a selection, we show Rated-Torque and Rated-Speed with each Motor.

There are more parameters for the Motor in the Database.

Select a Gearbox Manufacture and Gearbox Part Number:

The Gearbox selection-box is to the top and right of the dialog.

The box to its right should then list the gearboxes available from the Manufacturer

This is to the right of the Gearbox selection-box

To help you make a selection, we show Gear-Ratio and Rated-Torque with each Gearbox.

There are more parameters for the Gearbox in the database.

Note: You should check the parameters and your selection with your hardware supplier.

X and Y Axes

The Torque and Speed show four quadrants of Torque and Speed: ± Torque and also ± Speed .

Torque and Speed Graph Scales

The Torque and Speed graphs auto-scale to the maximum and minimum, even if the motor cannot deliver the Torque or Speed.

The Torque & Speed graphs scale to the capacity of the Motor.

Points in the Graph

Thus, when the Torque or Speed, or both, change rapidly, the Points are further apart.

Point Colours

The Points have a colour code. The colours indicate whether the Motor or Gearbox, or the two, have the Torque and Speed capacity to drive the mechanism.

Duty-Cycle Percentages

There are three Duty Cycle Plots

Each Duty Cycle Plot shows the distribution of the Power, Torque and Speed of the Motor for a complete Machine Cycle.

If you select 'Show Limit Line' check box, then you can see if the motor must perform with more than its rated or maximum capacity.

If you want more detail for each chart, then increase the 'Bin Count'. You may also need to increase the number-of-steps in the Machine Settings dialog.

Click the Edit DB (Database) button

There are Six tabs

Ts = Stall Torque [Nm]

Tr = Rated Torque [Nm]

Tp = Peak Torque [Nm]

Wr = Rated Speed [RPM]

Jm = Motor Inertia [kg.cm^2]

Ke = Voltage Constant [Vrms / 1000RPM]

Kt = Torque Constant [Nm/Amp]

R = Winding Resistance (Phase to Phase) [ohms]

L = Inductance (Phase to Phase) [mH]

Vmax = Maximum Voltage [V]

Step 1: Save the Old Database

Backup the original Database

This is typically in: C:\ProgramData\PSMotion\DBase\...

Step 2: Suppliers

2.	Click the '' , at the top

Step 3: Units

Check that the unit you will be using in your database is in the list of Units.

If the units you want to use are not, you can add a new Unit to the list.

2.	Click the '' , at the top

Step 4: Prepare the .CSV file.

The CSV file must include the text:

•	'Units' in Cell A3

Row 3 must include the Units identical in text to the units in the Units Units tab - see Step 3

Row 4 must have the Fields the same as the Column Header in the tab to which you will append your new data.

An example .CSV File, the image shows:

If you do not have the particular data, for a Part, for example, Ke, then add zeros ('0') to the column in the .CSV File.

Step 5: Click the worksheet tab in the database in to which you want to import the .CSV data

Step 6: Import the .CSV File

The data in your .CSV file will append to the data in the database.

Why is the Motor Torque different to the Torque given at the Pin-Joint when you Display Forces with the Display Force Vectors tool?

The Torque we show in the graphic-area when we 'Display Force Vectors' is the torque to move only the mechanism.

It does not include the Torque to move the motor and gearbox. It is necessary to add the Torque to accelerate the motor and the gearbox to Torque in the graphic-area.

Clearly, the Torque to move motor depends on the ....motor and gearbox.

Each motor and gearbox has its own inertia, and efficiency.

The higher the gear ratio, the less Torque the motor 'sees'. But the motor must run faster and accelerate more itself.. This influences the overall power, as well as system efficiency.

The performance characteristics of a brushless servomotor (motor/drive combination) are described by a torque/speed operating envelope.

As shown below, the coloured areas of the curve identify the Exceeded Duty, Continuous Duty and Intermittent Duty zones of the system.

Exceeded Duty

To move the mechanical system, the gearbox and itself, the servomotor must exceeds its maximum speed or the maximum torque, or both.

Continuous Duty Zone

The continuous duty zone is bordered by the maximum continuous stall torque up to the intersection with the intermittent duty line. The continuous torque line is set by either the motor’s maximum rated temperature, or the drive's rated continuous current output, whichever is less.

The system maximum continuous or 'voltage line' is set by the voltage rating of the drives, the line voltage supplied, and the motor winding.

The system can operate on a continuous basis anywhere within this area, assuming the ambient temperature is 40°C or less.

Intermittent Duty Zone

The intermittent duty zone is bordered by the peak stall torque line and the system voltage line. The peak torque line is set by either the drives’ peak current rating, which the drive can give for a limited time, or the maximum rated peak current for the motor, whichever is less. Higher torque levels may be achievable at higher power levels.

Peak Torque: (ԎPS ) The Peak Torque the Motor and Gearbox at Stall Speed

Continuous Stall Torque: (ԎCS ). The Continuous Torque the Motor and Gearbox can give Continuously at Stall Speed.

Maximum Speed: (ωMAX ) Maximum possible speed of the Motor and Gearbox. Not attainable when the voltage is limited by the drive.

Knee Speed: (ωK ) The Speed at 'knee' in peak envelope that is the intersection of the Peak Torque with the Voltage Torque/Speed Limit Line.

Motor Providers also give a Rated Power - this is dependent on the Drive.

Continuous Rated Torque:(ԎCR ). The Continuous Torque at the Speed of the Rated Power.

Rated Speed: (ωR ) The Rated Speed or Speed at Rated Power. The motor can operate at this speed with the supply voltage.

The Reflected Inertia at the Motor shaft is usually continuously varying in a machine cycle.

With Constant Inertia Mechanical Systems it is easy to calculate torque.

With mechanisms, the torque is dependent on reflected inertia that is a function of Acceleration, Velocity, and Position.

PSMotion has developed algorithms to calculate these, which we believe give a true indication of the reflected inertia at a motor shaft for even the most complex mechanisms.

The equations below, are calculated at every instant in a machine cycle.

Typically, you aim to make the Reflected Load Inertia = (Motor + Gearbox) Inertia.

However, when the Load Inertia is not constant, it is more difficult to select the Motor and Gearbox.

Speed:

ωm = N × ωL

αm = N × αL

Torque

Tm = - αm (JG + JM) + |TVD |

TL = (TMD / N ) / η

TT = Tm +  TL

N    = Gear Ratio

ωm = Motor Angular Velocity

ωL  = Load Angular Velocity

TT   = Total Torque

Tm  =  Motor Torque

TL  = Load referred to Motor Shaft.

TVD = Viscous 'Drag' Torque. It is added to the torque.

TMD = Torque derived by MechDesigner at the Motor Shaft ( f{PL , ωL, αL } )

JG   = Inertia of Gearbox

JM   = Inertia of Motor

αm  = Motor Acceleration,

αL =  Load Acceleration

η    = Gearbox Efficiency